This invention relates to a method of adding to a silica refractory structure in a working environment at a temperature in excess of 600.degree. C.
The expression "silica" is used herein in the sense used in British Standard 3446 to define "silica refractory" as a refractory material which, in the fired state, contains not less than 92% SiO.sub.2 by weight.
Principal uses of silica refractories are in steel furnaces, coke ovens, gas retorts and glass tank furnaces.
The invention may be used in the modification of an existing structure, for example in the building of a wall or duct to divert flue gases or for some other purpose, but it is presently believed that the major practical use of the present invention will lie in the field of repairing damaged structures, and the present specification will accordingly be directed mainly to the use of the invention in that way.
With the effluxion of time, silica refractory structures deteriorate for one reason or another, and they consequently require repair. Large furnaces take several days to cool to ambient from their working temperature, and they require a similar reheating time because the silicon dioxide in their structure, present in cristobalite and tridymite form is extremely sensitive to thermal shock at temperatures between 20.degree. C. and 600.degree. C. In particular, cristobalite is characterised by a crystalline inversion, generally between 200.degree. C. and 250.degree. C., which is accompanied by a change in length of about 1%.
It is accordingly desirable to effect any necessary repair while the silica refractory structure is hot. Unfortunately, the sensitivity of conventional refractory silica bricks to thermal shock effectively prevents their use in hot repair work unless they have been preheated. It will be appreciated that such preheating is also time-consuming.
It will be understood that it is necessary that a silica refractory wall should be repaired with silica refractory and not some other material in order to achieve compatibility, inter alia, of rates of expansion and thermal conductivity as between the repair and the original brickwork.
Hot repairs have in the past been carried out according to two distinct systems. In one such system, use is made of vitreous silica bricks. Vitreous silica has a very small coefficient of thermal expansion so that bricks at ambient temperature can be transferred immediately to the hot repair site without any substantial risk that they will crack due to thermal shock. The bricks are laid and their interstices are packed with granular refractory material to hold them in position. Such thermal expansion of the bricks as does take place further compresses the packing granules. Unfortunately, operating according to this system does not result in a very high quality repair, since the interstices between the vitreous silica bricks are not airtight. This is of very considerable importance in the case of coke ovens because of the different gas compositions inside and outside such ovens and is also important for example when repairing the roof of a glass melting tank furnace. Any flame which penetrates an interstice in the roof of such a furnace will rapidly erode the surrounding material so that further repair is soon required.
In the other such system, a mixture of finely divided particles of exothermically oxidisable material and particles of refractory material are projected against a surface and burned during projection so that under the heat of combustion a coherent refractory mass is formed on that surface. Particular examples of such processes are described in Glaverbel's British Patent Specification No. 1 330 894 and in copending British Patent Application No. 82 33 319 (Publication No. GB 2 110 200 A). Such processes can lead to highly effective repairs, but the rate of application of new material is not high, and where silicon is used as the or an exothermically oxidisable material (as is recommended or required in those specifications) the process is rather expensive especially for comparatively large repairs.